Each fraction separated in the refining of crude oil is a mixture of hydrocarbons. Identify a property of a hydrocarbon that results in its presence in a particular fraction. (b) Name the fraction, obtained from crude oil, that is used for jet aircraft. (c) What process is used to convert molecules in this fraction to smaller molecules that are in greater demand? (d) What are the two reference hydrocarbons used to assign octane numbers to fuels? (e) Give two advantages of adding oxygenates, such as ethanol, to petrol. (f) Ethanol can also be used as a fuel. Calculate the heat of formation of ethanol given that the heat of combustion of ethanol is −1370.7 kJ mol⁻¹ and that the heats of formation of carbon dioxide and water are −393.5 and −285.8 kJ mol⁻¹ respectively.

Leaving Cert Chemistry Fuel & Thermochemistry: Each fraction separated in the r

Each fraction separated in the refining of crude oil is a mixture of hydrocarbons - Leaving Cert Chemistry - Question 6 - 2015

Question 6

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Each fraction separated in the refining of crude oil is a mixture of hydrocarbons. Identify a property of a hydrocarbon that results in its presence in a particular ... show full transcript

Worked Solution & Example Answer:Each fraction separated in the refining of crude oil is a mixture of hydrocarbons - Leaving Cert Chemistry - Question 6 - 2015

Step 1

Identify a property of a hydrocarbon that results in its presence in a particular fraction.

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Answer

One significant property of hydrocarbons that influences which fraction they belong to is their relative molecular mass or boiling point. Hydrocarbons with lower molecular masses tend to be present in lighter fractions, while those with higher molecular masses are found in heavier fractions.

Step 2

Name the fraction, obtained from crude oil, that is used for jet aircraft.

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Answer

The fraction obtained from crude oil that is used for jet aircraft is kerosene, also referred to as paraffin.

Step 3

What process is used to convert molecules in this fraction to smaller molecules that are in greater demand?

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Answer

The process used to convert larger hydrocarbon molecules in kerosene to smaller, more desirable molecules is called catalytic cracking.

Step 4

What are the two reference hydrocarbons used to assign octane numbers to fuels?

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Answer

The two reference hydrocarbons typically used to assign octane numbers to fuels are heptane (which has a defined octane number of 0) and 2,2,4-trimethylpentane (which has a defined octane number of 100).

Step 5

Give two advantages of adding oxygenates, such as ethanol, to petrol.

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Answer

Adding oxygenates like ethanol to petrol reduces emissions of harmful pollutants, contributing to cleaner air quality.
Oxygenates enhance the combustion efficiency of fuels, leading to improved engine performance and reduced engine knocking.

Step 6

Calculate the heat of formation of ethanol given that the heat of combustion of ethanol is −1370.7 kJ mol⁻¹ and that the heats of formation of carbon dioxide and water are −393.5 and −285.8 kJ mol⁻¹ respectively.

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Answer

To calculate the heat of formation of ethanol ( ext{C}_2 ext{H}_5 ext{OH}), we can use the following thermodynamic relationship:

Heat of combustion = ∑(ΔH_f products) - ΔH_f reactants

The reaction can be expressed as:

ext{C}_2	ext{H}_5	ext{OH} + 3	ext{O}_2

ightarrow 2 ext{CO}_2 + 3 ext{H}_2 ext{O}

Using the heats of formation:

−1370.7 = [2(-393.5) + 3(-285.8)] - ΔH_f( ext{C}_2 ext{H}_5 ext{OH})

Calculating the right side:

−1370.7 = [−787.0 + −857.4] - ΔH_f( ext{C}_2 ext{H}_5 ext{OH}) −1370.7 = −1644.4 - ΔH_f( ext{C}_2 ext{H}_5 ext{OH})

Rearranging gives:

ΔH_f( ext{C}_2 ext{H}_5 ext{OH}) = -1644.4 + 1370.7 = -273.7 ext{ kJ mol}^{-1}

Thus, the heat of formation of ethanol is −273.7 kJ mol⁻¹.

Each fraction separated in the refining of crude oil is a mixture of hydrocarbons - Leaving Cert Chemistry - Question 6 - 2015

Worked Solution & Example Answer:Each fraction separated in the refining of crude oil is a mixture of hydrocarbons - Leaving Cert Chemistry - Question 6 - 2015

Identify a property of a hydrocarbon that results in its presence in a particular fraction.

Name the fraction, obtained from crude oil, that is used for jet aircraft.

What process is used to convert molecules in this fraction to smaller molecules that are in greater demand?

What are the two reference hydrocarbons used to assign octane numbers to fuels?

Give two advantages of adding oxygenates, such as ethanol, to petrol.

Calculate the heat of formation of ethanol given that the heat of combustion of ethanol is −1370.7 kJ mol⁻¹ and that the heats of formation of carbon dioxide and water are −393.5 and −285.8 kJ mol⁻¹ respectively.

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